An Advantage with a cost
by 25Dragonfly
Summary: Sans has always been able to extinguish different timelines. But, not long after the catastrophe with W.D Gaster, a certain human just happens to fall into the underground and becomes the future of the Underground. Sans now has to deal with knowing and experiencing three timelines, one of which the human is pacifist, and the other? They're complete genocidal manics.
1. Chapter 1

**~Prologue~**

* * *

Alphys slowly opens her eyes still heavy with sleep. She glances at the clock, only to see that it is three A.M.

"Ugh, stupid insomnia," she sighs, slowly climbing out of the warm dungeon that is her bed and trudging through the dark labratory in desprate search of a light switch.

"Oh, up already Dr. Alphys?" a rich, sort of demonic voice asks, lisping on his S's.

Alphys nods as her hand searches up and down the wall until she feels a switch and presses it, "uh-huh, insomnia again." She looks up at the man in front of her.

Light glints off of the green, shimmering scales that are untouched by a fine, white lab coat. Purple hair, longer than that of _Hatsune Miku's;_ who is something called a _Vocaloid_ that Alphys is a big fan of.

"The question should be, Gaster, what are you doing up so late?" Alphys questions. The Royal scientist grins to himself, as he places his webbed hand on the machine in front of him and lets out a sigh of contentment.

"Because, my dear Alphys. I have finished it." He says.

Alphys lets out a gasp as her eyes widen, "Y-you d-d-d-d-don't mean the D-d-d-d-d-determination Extractor d-do you?" She stammers in fear, "B-but Lord Asgore t-t-t-told us to wait a-a-a-a-a-a-a-a-a month to finish it.

Gaster turns around and looks at the shivering Otaku with his pale yellow eyes, "Why yes, yes he did. But we don't have to tell him right away now do we?"

"W-well I guess," she admits, "but, what if he comes down a-a-a-and wants to inspect the progress?"

The scaled, fish-like monster just grins at her, "well, we'll just say its too dangerous at the moment."

"O-okay," Alphys lets out a small sigh of relief, but tenses up again, "b-b-but how do we know that it works? I mean, we can't just test it out on unsuspecting monsters can we?"

"No," Gaster replies, "and we won't have too. I'll just use it on myself."

Alphys eyes widen with terror at the sudden declaration, "B-but G-G-G-Gaster! You can't do that! What if the King finds out? B-but most importantly what about your daughter, Undyne? About Sans and Papyrus?"

The royal Scientist laughs, "why of course, they're already here!"

Soon, a certain magical spear knocks down the door and three figures stomp in. Alphys immediately hides her face within five seconds of seeing Undyne.

"Hey dad!" Undyne greets Gaster before looking around to find Alphys, who has taken to hiding behind a half-full bag of dog food, "huh? Where's Alphys?"

"she's right here," a bored, and lazy sounding voice says plainly, pointing to the bag of dog food. Alphys' face immediately turns into a deep crimson red.

"S-Sans!" She shouts at the Skele-bro. Undyne giggles as her cheeks start to assume a light pink (if thats possible).

"Hey Alphys," she says calmly before turning to her father.

"SANS! GET BACK HERE AND PICK UP YOUR SOCK THAT HAS BEEN SITTING ON MY HEAD FOR FOUR WEEKS NOW!" Papyrus yells at his brother. Sans walks up to him, picks up the sock and then sets it back down.

"there, i did it."

Papyrus fumes before errupting like a volcano on steroids, 'THIS IS JUST LIKE THE SOCK IN THE LIVING ROOM! JUST FORGET ABOUT IT!"

"ok bro." sans says quietly.

"Okay, thats enough shenanigans for now," Gaster says, calming the group down, "now, everyone knows why they're here. So lets get started!" and with that he hits the switch on the Determination Extractor.


	2. Chapter 2

"SANS! WAKE UP! YOU CAN'T NAP AT YOUR STATION ALL NIGHT YOU LAZYBONES!" Papyrus shouts, relentlessly shaking his brother until he finally opens his eyesockets.

"what is it bro? a little sleeping ain't hurting anyone." Sans says groggily, as he flaps his hand lazily motioning for his brother to go away.

"YES IT IS! IT'S HURTING OUR CHANCE OF GETTING INTO THE ROYAL GUARD!" Papyrus stamps his foot against the snowy surface, only to end up slipping on the ice meters below.

Sans rolls his eyes, he couldn't care less about the Royal Guard "look bro! there's spaghetti over there!" he points to a point away from his station.

"THERE IS? GEE! YOUR EYES ARE GOOD! I'LL LEAVE YOU ALONE THEN!" and with that, Papyrus dashes into the woods in hope in finding the supposed plate of spaghetti that his brother completely made up.

"that was too easy," Sans murmurs to himself, letting his head drop back on his desk.

It doesn't take long before he feels his cell phone vibrating in his jacket pocket.

 _oh great. papyrus has found out that i lied to him just because i wanted a few hours more to sleep._ he thinks, picking up his phone and answering it.

"hello?" Sans says, completely expecting a complete lecture from his brother about how its rude to mislead people and blah blah blah. Sans doesn't even pay attention most of the time.

"Hello!" a sweet motherly voice responds. He doesn't know this lady's name, but they do exchange bad puns at a door all the time.

"hey. I sent my brother on a wild goose chase on some spaghetti in the woods because i wanted to sleep some more and I said you should go spa-GHET-ti your spaghetti!"

Soon the sound of spontaneous laughter can be heard through the receiver.

"That's not very nice, but it's awesome!" The lady laughs, "anyways the reason why I have called you is because yet another human has fallen into the underground. They might try to escape, and if they do, I want you, Sans, to be friends with them, and try not to kill them please! They are the seventh soul that the king is after, and they need to be protected!"

Sans nods his tired head. "will do ma'am."

"Thank you." and with that, the lady hangs up.

"this shouldn't be too bad-" he begins to say, but stops in his tracks as a vision blurs past him. It's faint, but he can make out spots of blood and corpses everywhere. He shakes it off.

 _might as well head closer to the door._

"SANS! YOU LIED TO ME! THERE IS NO SPAGHETTI OVER THERE IN THE WOODS!" Papyrus yells, stomping closer to the lazy skelebro, "EXPLAIN YOURSELF!"

"oops, sorry my bad," Sans says, shifting his eyes over to the left, "anyways, i'm headed towards the ruins. i'll see you here in a bit."

He turns around and walks towards a bush with a very narrow pathway made entirely of pure ice and skates to the other side.

"Hey there stranger! Would you like some Nice Cream? It's the frozen treat that warms your heart!" Some blue goat in a silly-looking red and yellow polo shirt advertises.

"i would, but i don't have a heart," Sans quickly turns to an imaginary camera and winks before going to the supposed "barrier" that Papyrus created to stop humans. Fortunately, he made the spars too far apart, so now anyone can pass through.

It doesn't take long before a human child arrives at the barrier wearing a purple stripped shirt.


	3. Chapter 3

Chapter two.

* * *

Alphys' POV.

* * *

 _Meanwhile in Alphys_ ' _Laboratory..._

"C'mon Naruto! C'mon! Beat the shit out of Sasuke, and then kiss him and tell him how you really feel!" I squeal, hugging my _Kyuubi_ plushie closer to my chest, as my eyes stay glued to the screen in front of me.

Soon, the screen fades, ending the episode.

"Oh c'mon!" I yell in disappointment. Now I have to wait yet another week before the next episode comes out. People say that _Naruto_ already ended up on the surface, and that it's slowly being leaked to the underground.

I nervously glance and my project that's half-finished. It's a special cream that I'm designing that keeps fish-like monsters from overheating in their armour.

"HEY! ALPHYS! LET ME IN! I'M OVERHEATING OUT HERE!" A familiar, "tough" voice yells, that follows with large BANGS!

I break out in a cold sweat. _Oh no! Oh no! Oh no! Oh No! There's anime stuff everywhere!_ _Undyne is going to think that I'm so uncool!_

"ALPHYS!" She repeats.

"I'm c-c-c-coming!" I stammer. Quickly shutting my laptop lid down, storing my little project away, putting on the lap coat to cover up my Death Note t-shirt, and rushing to the door.

"I-I'm s-s-sorry about the wait!" I open the door, while apologizing profusely.

"Sure took you long enough! It's cool though. I just need a break," Undyne says, "no need to get so damn nervous."

"Y-yeah, I-I guess n-not!" I murmur, trying to hide my blush with my lap coat.

"Hey! I didn't know that you liked Chobits!" She remarks. Looking at one of my manga.

 _Crap!_

"W-well, I-I-I'm just using that to study human behaviour," I deny, quickly snatching it away from her grip.

"Using manga to stud human behaviour? Thats awesome!" Undyne grins, "speaking of humans, I heard that you have been setting up security cameras all around the Underground lately just in case one falls down here."

"Well, uh- I guess. Yeah thats true."

"That's good," She slips into one of my beat-up couches and sighs, "hey, Alphys. If one does fall down here. Can you please let me know?"

I nod, "of course."

'Awesome!" She responds, grinning at me. I give her a small bashful smile as our gazes lock together.

Soon, my stomach growls, reminding me that it's been three days since I've eaten anything, and completely ruining the moment.

Undyne immediately bursts into laughter, "what was that? Some-sort of lion living inside of you or something?"

"I uhhhhhhhhh," I quickly hide into my lab coat, _Oh shit! Oh shit! Oh shit! Now she'll never think I'm cool, and probably thinks I'm a complete loser now!_

"Man your adorable!" she says, before stiffening up, "uhhhhh, I mean you r-really should eat something!"

"Y-yeah..."

Undyne the gets up from the couch and stretches, "well, I should be headed off. Papyrus's cooking lesson is in five hours and I should get ready," suddenly she walks over to me and hugs me.

"See ya around Alphy! Let me know if you see a human!" and with that, Undyne dones her armour and walks out.

I just stand there as the door closes.

Undyne just hugged me! Oh my god! I-i have to let the others know immediately!

* * *

Third Person PoV.

* * *

Undyne sighs, as she begins getting into her new armour. _What was I thinking back there? Calling her adorable and then hugging her? You're pathetic Undyne. Now she_ _knows, and is probably extremely creeped out! Now I have no chance in hell with her._ She chuckles at the event and blushes a little. _But, she is adorable! She's just like Tsukimi, adorable!_

She puts on her helmet and walks out of her house and begins to walk around Waterfall.


	4. Chapter 4

Sans quickly makes his way to the "barrier" Papyrus constructed to keep humans out, and hides behind a tree not far away.

It doesn't take long until he hears tiny little footsteps crunching in the fresh snow, and the sound of chattering teeth closing in on him. The figure stops immediately as soon as they reach the "barrier".

 _i guess i'll go do what i'm supposed to..._ he sighs, as the lazy skelebro gets up and walks behind the human.

"Is that anyway to treat a new friend? Now, turn around and shake my hand,"

The human turns around to face the strange voice behind him. Yellow skin, medium length brown hair, a blank expression on his face, and a blue sweater with purple stripes circled around their chest.

They reach out to shake the boney hand, but are greeted with a loud farting noise as the skeleton on the end howls with laughter.

"heh, the old whoopee couson in the hand trick. it never gets old," Sans winks, "hi, i'm sans. sans the skeleton. you're a human right?"

The human nods silently.

"thats hilarious. well... i'm supposed to be on watch for humans right now. but y'know. i don't really feel like capturing anybody at the moment."

The human looks at Sans with a confused look on their face.

"alright, come with me. don't worry. my bro built this so the spokes are wide enough for you to walk through. by the way, what's your name?"

"...Frisk." they say silently, as Sans grabs their hand and runs towards his sentry station.

"Okay, well then frisk, go hide behind that conveniently shaped lamp. my brother, papyrus, is going to stomp in here any moment now. and trust me, you don't want him to see you."

Frisk nods and quickly walks towards the lamp that was somehow built in their exact shape.

"SANS!" Soon, another skeleton? In a strange outfit composed completely out of plastic runs up to Sans with a pissed off look on his face, "ITS BEEN THREE DAYS AND YOU STILL HAVEN'T RECALIBRATED YOUR PUZZLES YET! YOU JUST HANG AROUND YOUR SENRY STATION WHAT ARE YOU EVEN DOING?!"

Sans looks off to the side. "nothin much, just lookin' at this cool lamp. c'mon take a look."

"NO! SANS YOU KNOW I DON'T HAVE THE TIME FOR THAT!" Papyrus stamps his foot against the ice, miraculously not slipping, "WHAT IF A HUMAN COMES TROUGH HERE! WE CAN'T JUST BE SITTING AROUND LOOKING AT LAMPS YOU IMBASOL! I WANT TO BE READY!"

"I WILL BE THE ONE! I MUST BE THE ONE! I WILL CAPTURE A HUMAN! THEN I,THE GREAT PAPYRUS WILL GET ALL THE THINGS A HEROICALLY DESERVE!" he stands in a somewhat narcissistic stance, "RESPECT, RECOGNITION, I WILL FINALLY BE ABLE TO JOIN THE ROYAL GUARD! PEOPLE WILL ASK TO BE MY FRIEND! AND I WILL ACCEPT IT WITH HUGE HONOR! I WILL BATHE IN A SHOWER OF WARM KISSES EVERY MORNING!"

"hmmm, maybe looking at this lamp will help ya." the older skelebro glances at Frisk before Papyrus begins having a fit.

"SANS! YOU ARE NOT HELPING! YOU LAZYBONES! ALL YOU DO IS SIT AROUND AND DO ABSOLUTELY NOTHING! YOU GET LAZIER AND LAZIER EVERYDAY!"

"hey, take it easy. i've gotten a ton of work done, a skele-TON!" Sans turns and winks at an imaginary camera as an annoying "da-dum-TISS!" plays in the background.

"SANS!"

"c'mon, you're smiling."

"I AM AND I HATE IT! SIGH... WHY DOES SOMEONE AS GREAT AS ME, HAVE TO DO SO MUCH JUST TO GET SOME RECOGNITION?"

"wow, sounds like you're working yourself... done to the bone!" Sans yet again turns around and shrugs at an imaginary camera with "da-dum-TISS!" playing in the back ground.

"UGH! I WILL ATTEND TO MY PUZZLES... AS FOR YOUR WORK? PUT A LITTLE MORE... BACKBONE INTO IT! NYEH-HEHEHEHEHEHEHEHEHEHEHEHEHEHEHEHEH!"

"HEH!"

Papyrus runs out from the area and into the woods in front of him.

"ok. you can come out now."

Frisk steps away and quickly runs over to Sans, "He's a little weird isn't he?" they ask in a small voice.

"heh, yeah. you better get going. if he comes back... you'll have to sit through more of my hilarious jokes."

Frisk quickly clings themselves to Sans' arm, and looks up at him with a scared look.

"whats the holdup? there's nothin' to be afraid of. it's just a dark cavern filled with skeletons and monsters." he winks.

Frisk nods and unhooks themselves before running off.

"also, this may be a bit of a stretch. but my brother has been feeling pretty down these past few days. so if you could maybe reveal yourself to him as a human, that would make a world of difference. thanks a bunch." Sans quickly calls out before seemingly dissapearing into thin air.


	5. Chapter 5

"hey, paps." i say, kicking around the snow right before the first puzzle "are all of your puzzles ready?"

"WHY OF COURSE BROTHER! THE GREAT PAPYRUS ALWAYS HAS HIS PUZZLES CALIBRATED!" He yells.

i chuckle "heh, of course paps."

"BUT YOU ON THE OTHER HAND! UGH YOU'RE SO LAZY! YOU WERE NAPPING IN YOUR BED ALL NIGHT!"

"uhhhhh... bro. i think thats called sleeping."

"THATS JUST AN EXCUSE!"

i just shrug before reaching into my jacket pocket, pulling out a bottle of ketchup and drinking the entire thing.

"SPEAKING OF PUZZLES, WHERE IS THAT HUMAN?"

I can hear quiet giggling in the background.

"HM? WHO GOES THERE?" Papyrus quickly turns around with his hand above his "eyes", "AH! THERE YOU ARE HUMAN!"

Frisk gives him a confused look and takes a small step forward.

"IN ORDER TO STOP YOU AND CAPTURE YOU, MY BROTHER AND I HAVE CREATED SOME PUZZLES! AND I THINK THAT YOU WILL FIND THIS ONE TO BE QUITE ELECTRIFYING! FOR THIS IS THE INVISIBLE ELECTRICITY MAZE! WHEN YOU TOUCH THE WALLS, THIS ORB WILL ADMINISTER A HEARTY ZAP! DOES THAT SOUND LIKE FUN?"

Frisk quickly freezes, and slowly backs away, "Uhhhhh... I'm sorry b-but-"

"BECAUSE THE CHANCES OF YOU HAVING ANY FUN, ARE ACTUALLY KIND OF SMALL..." Sweat quickly forms on top of Papyrus's head as he pats it off with his slightly torn up scarf.

 _awwwww. seems like bro really doesn't want to hurt anybody. not that i'm surprized or anything. but i can see why undyne won't let him into the_ _royal guard..._

"OK, YOU CAN GO AHEAD NOW!"

Frisk barely moves an inch before _ZZZZZZZZZZZZZZAP!_ Papyrus is immediately eletrecuted.

He stamps his foot on the ground angrily, "SANS! WHAT DID YOU DO!?"

i look off to the side, "uhhhh, i thnk the human is supposed to hold the orb..."

"OH! RIGHT!" He quickly walks through the puzzle, leaving a path of obvious footprints, "HOLD THIS PLEASE." he throws the orb onto the human's head and quickly walks around the puzzle. "OKAY! TRY NOW!"

Frisk giggles while they walk through the "maze" with ease.

"INCREDIBLE! YOU SLIPPERY SNAIL! YOU SOLVED IT SO EASILY! TOO EASILY... BUT! YOU SURELY WON'T BE ABLE TO PASS THE NEXT ONE! AS IT IS DESIGNED BY MY BROTHER! NYEH HEH HEH HEH HEH HEH HEH HEH HEH HEH HEH HEH!"

"Wow, your brother sure is silly," Frisk giggles, whilist cutely covering their mouth with their sleeve.

"yeah. he is a bone-ified lover of japes heh."

Suddenly, another image flashes before my eyes. Frisk is covered in a white powder, their clothes stained with blood, and they are holding a knife as their face is like a stone. No emotion whatsoever.

"Sans?" They say. I quickly blink as the scenery changes back to normal.

"what? oh, its nothing. i just... blacked out for a second there heh-heh." i wink at the kid as they laugh.

"Well, I'll be going!"

"alright. See ya kid."

* * *

i follow the kid around for a bit. Making sure that my visions are just delusions. And that seems to be that case here, they haven't hurt a single soul. Here and there they might attack someone, but they never kill.

It's actually been kinda fun watching them and Pap interact.

"HEY, BROTHER..." He begins. I snap out of my trance.

"what bro?"

"I'VE BEEN THINKING, THAT IF I EVER WANT TO CAPTURE THIS HUMAN, THEN I WILL HAVE TO FIGHT THEM."

I can feel my left eye burning up, "what did you just say?

"YOU HEARD ME BROTHER." Papyrus sighs as he puts his arm around me, "DON'T WORRY, I'LL BE FINE! THEY DON'T SEEM DANGEROUS IN THE SLIGHTEST! BESIDES, WE ALL KNOW THAT I, THE GREAT PAPYRUS CAN NEVER BE DEFEATED IN BATTLE!"

I chuckle and smile a bit, "of course bro."

"WELL, I GUESS I MUST GET GOING. DON'T WORRY SANS, I'LL BE FINE." He gets up from our ratty couch and walks out the front door.

 _might as well follow him just in case things get out of hand..._

I wait a couple minutes before heading out the front door. It doesn't take me very long to find them in a passageway just behind my house.

"UGH, WHATS TAKING THEM SO LONG?" I can hear Pap mutter to himself, I can faintly see Frisks figure immerge through the fog, "AH THERE YOU ARE HUMAN!"

 _papyrus, you better not fucking die. or else, this kid's going to have a bad time..._


	6. Chapter 6

the "fight" (if you can even call it that), goes on for about ten minutes. frisk hasn't attacked him at all, in fact they appear to be flirting with him. and now papyrus has invited them to a "date". admittedly, i was laughing... the entire time.

"WELL HUMAN! THE GREAT PAPYRUS AWAITS OUR DATE!" he announces, as frisk giggles, waves and walks away.

i wait until i feel as if they've gone far enough not to hurt anybody before letting out a sigh of relief. no need to step in this time.

"welp, i guess i should be headed home," i sigh, snapping my fingers and teleporting back to our house.

* * *

"AHHHHHHHHHHH! HUMAN! YOU ARE MEETING ALL OF MY STANDARDS!" i can hear pap yell from up in his room, as frisk laughs hysterically.

"FINE, I WILL ADMIT DEFEAT!" he gives in, "BUT ONLY THIS ONCE!"

"there ya go paps!" i yell. Soon, the door opens as the two walk out, frisk giggling uncontrollably.

"YELL, FRISK! I MUST GO DO SOME ERRANDS! I WON'T BE TOO LONG! YOU CAN SLEEP HERE FOR THE NIGHT!" papyrus announces, before running and jumping through the window.

frisk waves goodbye, before descending down the staircase and sitting down next to me.

"hey kid," i say.

"Hey Sans," they reply, yawning and covering their mouth with their over-sized sleeve.

"what? ya tired or somethin'?" i ask, looking over.

"Yeah... and hungry..."

"i hate to break it to you, but we just ran out of food. are ya going to be okay until tomorrow?"

"yeah..." they nod vaguely before they lean against me, gripping my jacket sleeve, "hmmm... you're warm. Which is confusing considering that you are a skeleton..."

"it's magic kiddo, us monsters are made of the stuff," I chuckle.

"It's strange. You monsters act more human than most of the humans I know on the surface."

"really?"

"Yeah..." their voice trails off as they fall asleep.

I take in a deep breath before getting up from my spot, placing a pillow underneath their head, switching off the tv and the lights, and taking off my jacket and placing it on their body.

"here ya go, you can have my jacket tonight Frisk, it's not like we have any blankets or nothin'. Welp, g'night kid." I peek a quick glance at them before climbing up the stairs, going up to my room and limply collapsing on my own mattress.

* * *

 _it's been at least an hour since papyrus has went to go fight the human, and it's starting to worry me. it doesn't normally take this long._

 _welp, might as well go check on him i guess..._

 _i snap my fingers, teleporting to the spot where papyrus said that they'd be._

 _i expect the sound of papyrus yelling "nyeh-heh-heh!", but all i'm greeted with is dead silence, i cant even hear the wind whistling._

 _"papyrus?' i call out, "papyrus! where are you?"_

 _oh no. nonononono! no! this can't be happening!_

 _"papyrus! this isn't funny anymore!"_

 _"PAPYRUS! JUST SAY SOMETHING THAT TELLS ME YOU'RE JUST KIDDING AROUND WITH ME! PLEASE!" i beg, but no response._

 _suddenly, I step into a pile of powder._

 _"what the hell?" i look down at my feet, and lo and behold I see pap's red scarf pinned down by the weight._

 _i drop to my knees, "oh no! no! no! no!" my hand reaches down and sifts through his dust before my finger touches the familiar fleece of his scarf._

 _my left eye darkens and my other one glows as I lift it up and I wrap it around my neck, it reeks of rotten spaghetti. "frisk, you are going to pay for this!"_

* * *

"PAPYRUS!" i scream. i look around to see the familiar walls of my room, and the sound of papyrus snoring loudly in the room beside mine.

"it was... just a dream. heh." i flop back down, "yeah... didn't feel like it though..."

insomnia kicks in just as i try and close my eyesockets.

sighing i get out of bed and i wonder down to where frisk is sleeping soundly. not an atom of dust or blood on them.

"kid..." i say, as a blue aura surrounds them and the float off of the couch.

 _wait a minute... what the hell am i doing?_ i wonder, as i gently let the kid down, "you better not kill anyone ya hear me?"

yawning, i walk back upstairs and i enter papyrus's room, and i climb into his bed, clutching his scarf as i drift off too sleep.


End file.
